File size limits in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the file size limit to 1MB using express.json().

Express

app.use(express.json({ limit: '[1]' }));

Drag options to blanks, or click blank then click option'

A1mb

B1024kb

C1000kb

D1MB

Attempts:

3 left

💡 Hint

Common Mistakes

Using '1024kb' (option B).

Using '1MB' (option D).

The correct way to specify 1 megabyte is '1mb' which equals 1MB in express.json() limit option.

2fill in blank

medium

Complete the code to limit file uploads to 5MB using multer.

Express

const upload = multer({ limits: { fileSize: [1] } });

Drag options to blanks, or click blank then click option'

A5000000

B5 * 1024 * 1024

C5242880

D5e6

Attempts:

3 left

💡 Hint

Common Mistakes

Using string values instead of number bytes.

Using 5e6 which is close but not exact 5MB in bytes.

Multer expects fileSize limit in bytes. 5MB equals 5242880 bytes (5 * 1024 * 1024).

3fill in blank

hard

Fix the error in setting the file size limit for express.urlencoded().

Express

app.use(express.urlencoded({ extended: true, limit: '[1]' }));

Drag options to blanks, or click blank then click option'

A1024kb

B1MB

C1000000

D1mb

Attempts:

3 left

💡 Hint

Common Mistakes

Using numeric values instead of string with units.

Using lowercase 'mb' (option D).

The limit option expects a string with size and unit, '1024kb' correctly sets 1MB limit.

4fill in blank

hard

Fill both blanks to set multer to accept files up to 2MB and only single file named 'avatar'.

Express

const upload = multer({ limits: { fileSize: [1] } });
app.post('/profile', upload.[2]('avatar'), (req, res) => { res.send('Uploaded'); });

Drag options to blanks, or click blank then click option'

A2097152

Bsingle

C2MB

Darray

Attempts:

3 left

💡 Hint

Common Mistakes

Using '2MB' string instead of bytes number.

Using upload.array() instead of upload.single() for single file.

File size limit is 2MB in bytes (2097152). To accept one file named 'avatar', use upload.single('avatar').

5fill in blank

hard

Fill all three blanks to create an express.json() middleware with 10MB limit, and multer to accept multiple files named 'photos' with 3 files max.

Express

app.use(express.json({ limit: '[1]' }));
const upload = multer({ limits: { fileSize: [2] } });
app.post('/upload', upload.[3]('photos', 3), (req, res) => { res.send('Files uploaded'); });

Drag options to blanks, or click blank then click option'

A10mb

B10485760

Carray

Dsingle

Attempts:

3 left

💡 Hint

Common Mistakes

Using '10MB' uppercase.

Using upload.single() instead of upload.array() for multiple files.

express.json() limit is '10mb' string. Multer fileSize limit is 10MB in bytes (10485760). To accept multiple files named 'photos' max 3, use upload.array('photos', 3).

Practice

1. What is the main purpose of setting a file size limit in an Express app using body parsers?

easy

A. To change the file type of uploads

B. To allow unlimited file uploads without restrictions

C. To automatically compress uploaded files

D. To prevent users from uploading files that are too large and slow down the server

Solution

1. Step 1: Understand file size limits in Express

   File size limits stop very large files from being uploaded, protecting server resources.

2. Step 2: Identify the main reason for limits

   Limits keep the app safe and fast by preventing overload from big files.

3. Final Answer:

   To prevent users from uploading files that are too large and slow down the server -> Option D

4. Quick Check:

   File size limits = prevent large uploads [OK]

Hint: File size limits protect server from big uploads [OK]

Common Mistakes:

• Thinking limits compress files automatically
• Believing limits allow unlimited uploads
• Confusing file type changes with size limits

2. Which of the following is the correct way to set a 1MB file size limit using Express's built-in JSON body parser?

easy

A. app.use(express.json({ sizeLimit: 1 }))

B. app.use(express.json({ limit: '1mb' }))

C. app.use(express.json({ maxFileSize: '1MB' }))

D. app.use(express.json({ limit: 1000 }))

Solution

1. Step 1: Recall Express JSON parser options

   The correct option to set size limit is 'limit' with a string like '1mb'.

2. Step 2: Check each option's syntax

   Only app.use(express.json({ limit: '1mb' })) uses 'limit' with correct string format '1mb'. Others use wrong keys or units.

3. Final Answer:

   app.use(express.json({ limit: '1mb' })) -> Option B

4. Quick Check:

   Use 'limit' with string size in Express [OK]

Hint: Use 'limit' option with string size like '1mb' [OK]

Common Mistakes:

• Using wrong option names like sizeLimit or maxFileSize
• Passing number without units as limit
• Using uppercase units incorrectly

3. Given this Express code snippet, what happens if a client uploads a JSON body larger than 500kb?

app.use(express.json({ limit: '500kb' }));
app.post('/upload', (req, res) => {
  res.send('Upload successful');
});

medium

A. The server throws an error and does not call the route handler

B. The server ignores the size limit and processes the full upload

C. The server truncates the JSON to 500kb and processes it

D. The server accepts the upload and responds with 'Upload successful'

Solution

1. Step 1: Understand Express JSON parser behavior with limits

   If the JSON body exceeds the limit, Express throws a 'PayloadTooLargeError' and skips the route handler.

2. Step 2: Analyze the code flow

   The route handler sends 'Upload successful' only if parsing succeeds, which won't happen if size is too big.

3. Final Answer:

   The server throws an error and does not call the route handler -> Option A

4. Quick Check:

   Exceeding limit causes error, no handler call [OK]

Hint: Uploads over limit cause error, no success response [OK]

Common Mistakes:

• Assuming server truncates large JSON
• Thinking handler runs despite error
• Believing limit is ignored

4. Identify the error in this Express setup that tries to limit JSON body size to 2MB:

app.use(express.json({ limit: 2 * 1024 * 1024 }));

medium

A. The limit value should be a string like '2mb', not a number

B. The limit option is not supported by express.json()

C. The multiplication is incorrect; it should be 2 * 1000 * 1000

D. The limit should be set in bytes as a Buffer, not a number

Solution

1. Step 1: Check express.json() limit option type

   The 'limit' option expects a string with units like '2mb', not a raw number.

2. Step 2: Analyze the given code

   Passing a number causes Express to ignore or misinterpret the limit, so it should be '2mb'.

3. Final Answer:

   The limit value should be a string like '2mb', not a number -> Option A

4. Quick Check:

   Limit must be string with units, not number [OK]

Hint: Use string with units for limit, not number [OK]

Common Mistakes:

• Passing number instead of string for limit
• Misunderstanding unit conversion
• Thinking limit option is unsupported

5. You want to limit file uploads to 3MB using the multer middleware in Express. Which code snippet correctly sets this limit and handles errors to inform users when files are too large?

hard

A. const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } });
app.post('/upload', upload.single('file'), (req, res) => {
res.send('File uploaded');
});

B. const upload = multer({ limit: '3mb' });
app.post('/upload', upload.single('file'), (req, res) => {
res.send('File uploaded');
});

C. const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } });
app.post('/upload', (req, res) => {
upload.single('file')(req, res, err => {
if (err) return res.status(400).send('File too large');
res.send('File uploaded');
});
});

D. const upload = multer({ fileSizeLimit: 3 * 1024 * 1024 });
app.post('/upload', upload.single('file'), (req, res) => {
res.send('File uploaded');
});

Solution

1. Step 1: Set file size limit correctly in multer

   The correct option is 'limits: { fileSize: size_in_bytes }'. const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } }); app.post('/upload', (req, res) => { upload.single('file')(req, res, err => { if (err) return res.status(400).send('File too large'); res.send('File uploaded'); }); }); and A use this correctly.

2. Step 2: Handle errors to inform users

   const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } }); app.post('/upload', (req, res) => { upload.single('file')(req, res, err => { if (err) return res.status(400).send('File too large'); res.send('File uploaded'); }); }); wraps upload.single in route handler with error callback to catch file size errors and respond properly.

3. Step 3: Compare options

   const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } }); app.post('/upload', upload.single('file'), (req, res) => { res.send('File uploaded'); }); lacks error handling, B uses wrong option 'limit', D uses invalid 'fileSizeLimit'.

4. Final Answer:

   Option C correctly sets limit and handles errors to inform users -> Option C

5. Quick Check:

   Multer limits + error callback = const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } }); app.post('/upload', (req, res) => { upload.single('file')(req, res, err => { if (err) return res.status(400).send('File too large'); res.send('File uploaded'); }); }); [OK]

Hint: Use limits.fileSize and error callback to handle large files [OK]

Common Mistakes:

• Using wrong option names like limit or fileSizeLimit
• Not handling errors to inform users
• Assuming multer auto-handles file size errors